Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility may be improved by providing noise cancelling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
An active noise cancellation (ANC) system achieves the suppression of noise by observing the ambient noise with one or more microphones and processing the noise signal with filters to generate an anti-noise signal, which is then played through a loudspeaker. The application of active noise cancellation to personal audio devices such as wireless telephones and headphones is intended to enhance the users' listening experience with respect to intelligibility and isolation from the ambient noise. Because the acoustic environment around personal audio devices may change depending on the noise sources that are present and the position or fitting condition of the device itself, an active noise cancellation system may be implemented with adaptive filters in order to adapt the anti-noise to take such environmental changes into account.
A stereo headset that provides active noise cancellation to a user may not always provide maximum ambient noise attenuation. Such scenario may be due to a number of reasons, including but not limited to, a fit of the headset to a user's ears, spectral content of the ambient noise, and manufacturing variations of transducer response. A fully adaptive ANC system may compensate for these factors to some extent by utilizing the user selected playback as a training signal to adjust ANC filters. Because the system has no control over what the training signal provided by the user will be, including volume level and spectral content, precautions must be taken by the system to ensure stable adaptation of the ANC filters. Consequently, a fully-adaptive system only allows adaptation during optimal conditions with the wearer having no knowledge of adaptation status. As a result, some systems exist which provide a forced adaptation option to the user which uses a preprogrammed training signal to adjust filter coefficients and/or filter gains. However, such forced adaptation is generally fixed based on what has been pre-programmed Forced adaptation may result in misadapted ANC filters and cause a negative experience for the user.
U.S. Pat. No. 9,142,207 (the “'207 Patent”), which is incorporated by reference in its entirety, contemplates that adaptive ANC systems may require protection against misadaptation. The protection contemplated in the '207 Patent involves identification of ambient noise or ambient events that are likely to cause the ANC system, upon adaptation while such signals are present, to generate undesirable anti-noise. In the '207 Patent, the adaptive ANC system disclosed therein is continuously adaptive, seizing all potential opportunities for adaptation in order to provide the most desirable antinoise to the listener's ears at all times. The underlying strategy of such a system is to continuously adapt, except when ambient conditions and events might cause adaptation to lead to undesirable antinoise. However, one cost of such a continuously adaptive system is battery drain—adaptation requires significantly more power than merely providing an ANC effect with a fixed (non-adaptive) antinoise filter.